Currently, phase shift mask (PSM) techniques are being developed and refined to extend the resolution of optical lithography in fabrication of integrated circuits. Through the use of PSM techniques, optical lithography processes are able to fabricate discrete devices with feature sizes that are below the illumination wavelength of an optical lithography illumination source.
One PSM technique known in the art is alternating phase shift mask (altPSM) lithography. AltPSM lithography is a two exposure lithographic technique. That is, a first lithographic exposure is complimented with a second lithographic exposure into the same undeveloped photo resist to create an incoherent sum of two images that resemble a target image of an integrated circuit layout. AltPSM lithography requires that masks of opposite phase values be placed on either side of a feature to be patterned.
One concern in altPSM lithography is maintaining the placement of phase shifters so that the phase shifters have opposite phase values that are 180° out of phase throughout the layout of the integrated circuit. In a layout having two adjacent phase shifters of the same phase value, a condition known as phase conflict arises. When a phase conflict arises, it is an indication that an alternating mapping of PSMs across each feature to be patterned using altPSM is not present. To resolve a phase conflict condition, the mapping of PSM's across each feature to be patterned must be revised to allow for the required alternating mapping of the PSM's across each feature.
AltPSM is typically used to form the gate of a metal oxide semiconductor (MOS) device. Unfortunately, when the MOS device is a transistor designed for use as a MOS capacitor, such as a decoupling capacitor, phase conflict arises due to the placement of the source region and the drain region in the center region of the polygon cell forming the MOS capacitor. Thus, a significant amount of die area in a layout of an integrated circuit is consumed by MOS capacitors.